1. Field of the Invention
The present invention relates to a system for leveling a specimen, such as a substrate, to be orthogonal to the optical axis of an imaging system.
2. Description of the Related Art
Many optical systems for inspection of specimen, such as substrates, exist in the prior art. One such system is described in U.S. Pat. No. 6,621,275, which is assigned to the current assignee and which is incorporated herein by reference in its entirety. Such systems can be used for inspection or testing and debug of semiconductor substrates or integrated circuits. Examples of systems for imaging flip-chip type integrated circuits through the silicon substrate are described in U.S. Pat. Nos. 5,208,648, 5,220,403 and 5,940,545. FIGS. 1A and 1B depict an example of an integrated-circuit testing system that is helpful for the understanding of the present invention.
FIGS. 1A and 1B are general schematics depicting the major components of the testing and debug system according to the prior art. The system operates in conjunction with a commercially available automated testing equipment 105 (ATE). The ATE generally comprises a controller, such as a pre-programmed computer 181, and a test head 124, which comprises an adapter 125 used to deliver signals generated by the controller 181 to the device under test (DUT) 110 (FIG. 1B) in a manner well known in the art. Specifically, the ATE is used to generate signals that stimulate the DUT to perform various tasks, as designed by the chip designer to check and/or debug the chip. The various signals generated by the controller 181 are delivered to the DUT via the adapter 125. The adapter 125 may include a space transformer, a DUT load board 126 and a DUT socket, in a manner well known in the art.
In the embodiment depicted in FIGS. 1A and 1B, the ATE test head is placed on top of a vibration isolated test bench 115, while the chamber 100 that houses the entire optics, imaging and sensing system, and an X-Y-Z stage 175, is situated below. The ATE is used to place the DUT from above, so that it is visible to the optics 120 (including an objective, that is not shown) via opening 185. Stage 175 enables placing of the collecting optics 120 at any locations within the opening 185.
The various elements of the system of FIGS. 1A and 1B will now be described with reference to their operational modes. Generally, the system operates in two modes: navigation mode and detection mode. In the navigation mode, switchable mirrors 135 and 165 are placed out of the optical path, as shown by the broken lines in FIG. 1B, so that the illumination source 130 illuminates the DUT 110. The light that reflects back from the DUT 110 is passed through the beam splitter 160 and onto the imager 145. The imager 145 can be any two-dimensional detector capable of imaging the DUT 110, such as, for example, vidicon camera, or a InGaAs focal plane array. In this manner, an image of the DUT is acquired.
A mechanized aperture 170 is provided at the image plane of the collection optics 120, together with field lens 195. In this particular example the entrance pupil of collection optics 120 is imaged by the field lens 195 onto the entranced plane of the focusing element of the detector in imager 145. In one implementation (not depicted here) the pupil entrance of the collection optics is imaged by the focusing element onto a fiber-optics which couples the collected photons into the detector in imager 145. According to this feature, the illumination path takes place through the mechanized aperture 170 (which is positioned at the image plane of the collection optics) and thereby its opening defines the field-of-view on the sample or device under test. The aperture also defines the portions of the sample imaged onto the imager 145. That is, depending on the particular test to be run, one may wish to select any particular section of the DUT for emission. Using information about the chip design and layout stored in CAD software, such as, for example, Cadence, and using navigation software, such as, for example, Merlin's Framework available from Knights Technology, one may select a particular device for emission test, and block the image and emission of the other devices in the field-of-view of the collection optics. When the user selects a device or location, the system activates the stage 175 so that the collection optics is centered on the selected device or location. Alternatively, as long as the area of interest is in the field-of-view of the collection optics, one can isolate the area of interest with the apertures and proceed to image and detect “selectively”. Then, the aperture 170 may be adjusted to increase or decrease the field of view as appropriate for the particular test desired.
When an appropriate field of view has been set and an image obtained, mirror 135 is rotated so that the light path towards the IR sensitive detector 150 is established (opened), as depicted by the solid line drawing. Additionally, light source 130 is shut off or blocked during testing. It should be appreciated, of course, that chamber 100 prevents any external light from reaching any of the optics, imagers and sensors enclosed within.
Photon sensing during testing is done by detector 150, which is, for example, an infrared sensor, such as a photomultiplier tube (PMT), a photocathode coupled to a multi-channel plate (MCP), an avalanche photodiode (APD), etc. In this configuration the ATE is sending testing signals to the DUT and, as the various devices inside the DUT switch state they emit light. The light is collected by optics 120 and is directed to the photodetector 150, which converts the optical input into an electrical output signal. The output signal from the detector 150 is sampled by the high-speed data acquisition electronics 155. Controller 180, which may be a general-purpose computer running dedicated software, is used to control the various elements of the system, such as the actuators and stages and sampler. The controller 180 receives sync signals from the ATE 105.
Another optional feature described in the prior art is the use of a laser illumination of the DUT. The laser feature can be used as a “laser pointer” to allow pointing to a device of interest. During navigation, light source 130 is activated and mirror 135 is swung out of the optical path position (dashed line), so as to illuminate the DUT. Light reflected from the DUT is then passed through the half mirror 160 and is imaged by the imager 145. Once an image of an area of interest on the DUT is obtained, minor 165 is swung into the optical path position. Then, laser source 140 is activated to create a laser beam and illuminate the DUT. Laser light reflected from the DUT is imaged as a relatively small “laser pointer” image by the imager 145. The stage can then be actuated and moved until the “laser pointer” points to a device of interest. Once that is achieved, laser source 140 is turned off and mirror 135 is swung into the optical path (solid-line) position. In this position, the optical path to detector 150 is aligned onto the same device previously illuminated by the “laser pointer” so that it can be emission tested.
Also shown in FIG. 1B is a solid immersion lens (SIL) 190, being part of the optics 120. The optics 120 includes an objective lens (not shown) which is housed in an objective housing. The use of a SIL enables a more efficient collection of photons, especially in conditions where the emission is very faint. When using a SIL, it is a normal practice to place the SIL in physical contact with the object to be inspected.
The system depicted in FIGS. 1A and 1B and described above is provided as an example of an optical inspection and/or testing system so that a better understanding of the invention can be had. However, it should be appreciated that optical systems of other designs can be improved using the invention described and claimed herein.